1. Field of the Invention
This invention relates to plant husbandry and, more specifically, to a method for controlling the abscission of plant parts such as stems, leaves, flowers and fruits. The need to control abscission arises in a wide variety of applications ranging from agriculture to ornamental plant cultivation.
A major concern in agriculture is the maximization of crop yield, especially in areas where suitable land is limited. Thus, it is desirable in orchards, for example, to delay the abscission of fruit and thereby prevent preharvest dropping. The harvest period may consequently be stretched to reduce the work pace and to minimize the quantity of harvested produce needed to be stored prior to shipment. Similarly, the number of pod initiations generated in leguminous crops, such as soybeans, may be greatly increased by delaying the abscission of its flowers to maximize total flower pollinization.
On the other hand, crop yields may be increased by timely promoting the abscission process. Mechanically harvested fruit, for example, which is too firmly attached to the plant may leave a firmly attached piece behind on the stem and be subsequently susceptible to contamination. It is, therefore, desirable just before harvest to "loosen" the fruit by controllably promoting the abscission process.
In addition to crop production, controllable abscission of ornamental plants is desirable. The promotion of leaf abscission may increase the density at which, and enhance the ease with which, the plants may be stored or transported. In contrast, however, it is desirable to retard the abscission of the petals of cut flowers to increase their allowable storage time prior to sale or use.
2. Description of the Prior Art
Abscission regulators of the prior art have consisted of a wide variety of chemical sprays, the composition of which differ with plant type and in accordance with the desire to delay or promote the abscission process.
There are many disadvantages associated with the use of chemical abscission agents. Many leave potentially harmful residues on the plant, the fruit, and in the soil, and some such as maleic hydrazide, dichlorophenoxyacetic acid, and 2, 4, 5-trichlorophenoxyacetic acid are highly toxic.
The chemical agents are vulnerable to inclement weather, and require a dry period in which to penetrate the plant or leaf skin if respraying is to be avoided. Because many are not biodegradable, residual effects of the regulators are carried for a long period in both the soil and plant system, and the levels of chemical residue may increase with repeated use of the agent.
Most chemical abscission promoters which remain in the soil additionally retard the subsequent seasons' growth of plants by retarding root formation and inhibiting the uptaking of nutrients. Additionally, the aging process of subsequently formed leaves is enhanced so that plant growth is further inhibited by premature defoliation. Finally, by prematurely stimulating bud formation, these promoters cause premature blossoming on plants having late forming buds which would otherwise develop the following season. Thus, flowers which would normally form in Autumn and provide fruit during the following growing season are now initiated as early as Spring, to the detriment of the following growing season's yield. Yield is further reduced by the inadvertent abscission of shoots which thereby reduce the quantity of born fruit.
Crops treated with chemical abscission promoters may undergo abscission before true maturity. In the chemical hastening of the ripening process of tomatoes, for example, the color is changed by the stimulation of certain enzymes, but the tomato remains high with alkaloids, generating a bitter taste and decreasing its shelf life.
The nutritional value of the chemically treated crop may also be detrimentally affected. A decrease in the calcium content of apples has been attributed to the lowering of soil pH by chemical abscission agents. It may be noted that the firmness of the fruit, as well as its nutritive value, depends on its calcium content, so that the quality of the fruit, as well as its ability to remain on the tree until full maturity, are both adversely affected.
Finally, the acidic nature of some chemical abscission agents create lesions and extensive peel injury on the fruit, particularly to citruses.
In summary, it may be seen from the foregoing that such agents often interfere with normal plant development and growth.
It might be contended that "Further Studies in Photoperiodism . . . ", Garner et al. Journal of Agric. Res., U.S.D.A., March 1923, pp. 871-3, 901-9, 914-19 (copy in art unit 337) might be relied on to teach the promotion and inhibition of abscission by illumination of plants "during the night period". The subject matter of such reference concerns the absicssion process which occurs just prior to the dormant stage of a plant. That abscission process is dependent upon photosynthesis by the plant and is triggered by the length of daylight and temperature. The reference artificially varies daylight length via broad spectra lighting to determine the effect upon leaf loss. As is known in the art, shorter days preclude decidious plants from deriving sufficient nutrients via photosynthesis. Consequently, the leaves are shed in order to contain nutrients within the more vital plant areas and, typically, the plant enters a dormant period.
In contrast to the reference, the invention herein relates to the "physiological" abscission process, after which active cell division in the abscission zone continues. Physiological abscission is related to hormone changes within the plant, rather than losses of nutrients. One may contrast the two types of abscission by the fact that the reference, in lengthening the daylight period, alters and extends the growth of the plant by keeping it from reaching its dormant state. By contrast, the present invention inhibits, or prolongs physiological abscission without altering the natural growth limitation of the plant. For example, extension of the daylight period will not prevent apple trees from dropping their apples. The tree will simply not enter dormancy at its usual time.
By its extension of daylight, the Garner et al. reference does not utilize narrow bandwidth lights having predominate spectral emission of the wavelengths indicated herein. There is certainly no suggestion that low intensity illumination may produce the results described herein. Finally, the reference does not suggest the use of a night interruption technique whereby the plant is illuminated subsequent to the initiation of the dark period.